1. Field of the Invention
The present invention generally relates to a technology for controlling a communication path, and specifically relates to a technology for controlling a communication path by relaying a frame including a transmission-source address and a transmission-destination address.
2. Description of the Related Art
In recent years, a technology such as a synchronous digital hierarchy (SDH) and an asynchronous transfer mode (ATM) has been mainly used in a core network having broad bandwidth and high reliability (for example, an Internet service provider (ISP) and a contents distribution network (CDN)).
However, with a technical innovation, because a variety of local-area-network (LAN) devices have become available at a moderate price, which makes a majority of end users including major enterprise users employ a LAN using an Ethernet (registered trademark) and an Internet protocol (IP), an introduction of the Ethernet (registered trademark) and the IP is rolling forward.
In such a network, a path control apparatus that controls a communication path of a frame, such as an Ethernet (registered trademark) switch and an IP router, is taking a critical role. The path control apparatus reads out a transmission-source address and a transmission-destination address stored in a header of a frame, and controls the communication path appropriately so that the frame makes it to a desired transmission-destination address. The path control apparatus generally maintains a correspondence relation between an address and a port through which a frame is input or output in a table format. When a frame is input, the path control apparatus acquires a port corresponding to the same address as the transmission-destination address stored in the header of the frame from the table, and controls the communication path by outputting the frame to the acquired port.
The correspondence relation between the address and the port in the path control apparatus is learned by associating a transmission-source address of a frame at a time when the frame is input and a port at which the frame arrived. Then, if another frame is not input again from the same transmission-source address within a predetermined time since the correspondence relation has been learned, the correspondence relation for the transmission-source address is deleted from the table because the reliability of the correspondence relation is decreased. This process of deleting the correspondence relation is generally referred to as an aging process.
In the aging process, because a correspondence relation between a specific transmission-source address and a port is deleted when a predetermined time has been elapsed since the latest time at which a frame had arrived from the specific transmission-source address, it is necessary to calculate a time elapsed from the latest frame arrival time for all of the correspondence relations as needed, which may result in an increase of the processing load. To cope with the problem, Japanese Patent Application Laid-open No. 2006-74554 discloses a technology for deleting the correspondence relation efficiently, in which a bit string of the same size is provided for each of the correspondence relations, a specific bit indicating a frame arrival in the bit string is shifted at a predetermined cycle, and when the specific bit is shifted to the end of the bit string, a correspondence relation corresponding to the bit string is deleted.
The number of correspondence relations (entries) that can be registered in the table is fixed, because the table maintained by the path control apparatus has a limited size. Therefore, when a large number of frames are input to the path control apparatus, it can happen that there is no vacancy in the entry of the table, which leads to a situation requiring an overwriting of the correspondence relation registered to any one of the entries.
However, when overwriting the correspondence relation, it is not easy to select an entry to be overwritten with fair and efficiency. In other words, if a correspondence relation having a transmission-source address from which a frame is input at frequent intervals is deleted when overwriting the entry, it will cause the overwriting of the entry to be occurred too often. As a result, the efficiency of the overwriting of the entry becomes worse. Although it can considered to be another method to overwrite a correspondence relation having the longest time elapsed since the latest frame arrival time, in this case, it is necessary to calculate the time elapsed for all of the entries, resulting in an increase of the processing load and the processing time.
If the table of the path control apparatus is not updated efficiently, because a case in which the same address as a transmission-destination address of a frame is not registered to the table occurs frequently, there will be a limit in the control of the communication path of the frame, resulting in a decrease of the throughput of the whole network.